Have a way to test throughput of snowflake proxy

A common question from snowflake proxy volunteers is whether or not their proxy is working (see comment 11 on #31109 (moved)). It would be great to have some kind of bandwidth test for proxy owners to see whether or not their proxy is reachable from a remote probe point. This might also help us find and diagnose problems with existing proxies.

Some notes:

• we can't ask the broker to assign us a specific proxy at the moment so this test would likely be separate from the broker (unless we add an entirely new feature which I'm hesitant to do)

• we'll have to protect this service from abuse somehow, probably by rate-limiting. See some discussion on #31874 (moved). It would be best to engineer a way so that only a proxy owner can run the test on their proxy.